1. Field of the Invention
This invention relates to plasma treatment equipment.
2. Description of Related Art
Manufacturers of integrated circuit devices commonly employ plasma treatment equipment. Such equipment generates a plasma containing reactants and then exposes a surface of a semiconductor wafer to the plasma reactants. Plasma reactants can etch away portions of a wafer exposed by a mask to form a patterned structure or remove layers of a wafer to thin the wafer. During such etching, the rate and uniformity of the etching process need to be within expected ranges. Otherwise, defects may result from overetching or underetching portions of the integrated circuits being manufactured.
One type of plasma treatment system generates a plasma stream that can be directed at an object being treated. U.S. Pat. No. 5,474,642 describes a plasma treatment system that uses a single jet from a plasma burner to form a plasma stream directed at a wafer. However, greater flexibility and uniformity may be achieved in a system that combines a pair of plasma jets to form a combined plasma stream. This type of plasma treatment equipment is described in U.S. Pat. No. 5,489,820 and an article entitled xe2x80x9cApparatus for Plasma Flow Monitoringxe2x80x9d at pages 72-78 in the book entitled xe2x80x9cEquipment for High Efficiency Technologies,xe2x80x9d Scientific and Production Association xe2x80x9cROTORxe2x80x9d, Cherkassi, USSR (1990). (The previously quoted article and book titles are translations of Russian titles.) In such systems, the direction, cross-section, energy profile, and composition of the combined plasma stream need to be within desired limits for a particular treatment. However, environmental factors such as magnetic fields, gas flows and movement of the objects being treated and deterioration or variations in the operating parameters of the plasma burners tend to shift the paths or directions of the plasma jets. These factors are difficult to predict or directly control. Accordingly, known plasma treatment systems have monitored the combined plasma stream and attempted to adjust the input parameters to keep the combined plasma stream within required limits.
FIG. 1 shows plasma equipment such as described in U.S. Pat. No. 5,489,820. That equipment includes two plasma generators or burners 1, an electric drive 3, magnetic circuits 4, solenoids 5, a power supply 6, gas supply systems 7 and 8, a recording unit 9, and a processing unit 10. Supply systems 7 and 8 provide gases to plasma burners 1, and from the gases, plasma burners 1 produce two separate plasma jets. The plasma jets converge to form a combined plasma stream 2. Electric drive 3, on which plasma burners 1 are mounted, permits adjustment of the separation and the angle between burners 1 to thereby adjust the paths of the plasma jets. The solenoids 5 and magnetic circuits 4, associated with burners 1, provide magnetic fields for further adjustment of plasma jets. In particular, power supply 6 under control of processing unit 10 supplies current to solenoids 5 to adjust the plasma jets that form combined plasma stream 2. Recording unit 9 measures a property of combined plasma stream 2, and based on the measurements from recording unit 9, processing unit 10 determines appropriate settings for electric drive 3, power supply 6, and gas supply systems 7 and 8. Further description of the elements in FIG. 1 can be found in U.S. Pat. No. 5,489,820, which is hereby incorporated by reference in its entirety.
A disadvantage of the system of FIG. 1 is the need to identify the appropriate system settings based on the combined plasma stream 2. In particular, a deviation in plasma stream 2 might arise from a number of different factors, and choosing an appropriate setting to correct the deviation may be difficult. These difficulties increase with the number of inputs to the combined plasma stream. Additionally, if reactants from a cold stream are added to the combined plasma stream, the reactants can disturb the shape of the combined plasma beam and upon becoming a plasma may glow much more brightly that the plasma from the original jets. Accordingly, addition of cold jets makes it difficult to identify the properties of the original jets from measurements of the combined plasma stream. Plasma equipment is needed that is able to configure multiple input systems to provide a consistent plasma stream despite variations in environmental factors and variations in operating parameters.
In accordance with an aspect of the invention, a plasma treatment system separately measures input plasma jets before the plasma jets merge into a combined stream. One embodiment of the invention measures the position of plasma jets in a plane upstream of where the jets merge into the combined stream. The positions are measured relative to a fixed reference, and particularly in a system that combines plasma jets with a cold jet, the positions of the plasma jets are measured relative to the injector of the cold jet. Since the plasma jets are directly measured the plasma jets can be more easily steered into the proper paths that provide a combined stream with the desired properties.
One advantage of monitoring the positions of the individual plasma jets and not the combined plasma stream is that the individual jets have structures that are simpler than the structure of the combined plasma stream. For example, the brightness distribution of the total plasma stream typically has a xe2x80x9cdouble-humpxe2x80x9d curve, with one hump contributed by each jet. The brightness distribution of the total plasma stream and hence monitoring and controlling of the combined stream are thus more complicated than for a single jet. Further, separate measurement of jets facilitates injection of a reagent into the combined plasma stream at a point where the jets merge into the combined plasma stream. The reagent affects the temperature of the total plasma stream, and may change the brightness distribution, ion concentration, spectral radiation factors, and heat flow. The reagent (i.e., the cold jet) also interacts with the jets aerodynamically, changing the cross-sectional dimension of the total plasma stream. With or without the reagent, the brightness distribution, the ion concentration, the spectral radiation factors, the heat flow, and the cross-sectional dimension of the total plasma stream are more difficult to control than are the positions of the separate jets.
One specific embodiment of the invention is a plasma apparatus that includes first and second plasma burners, a measurement system, and a processing and control system. The first plasma burner generates a first plasma jet. The second plasma burner that generates a second plasma jet that is directed to join with the first plasma jet in a combined stream. The measurement system is positioned to separately measure the first plasma jet and the second plasma jet. In operation, the processing and control system determines at least one characteristic such as the position, cross-section, energy, or composition of the first plasma jet and a similar characteristic of the second plasma jet. Based on those determinations, the processing and control system adjusts the first and second plasma jets so that the characteristics of the first and second jet match predetermined characteristics that provide the combined stream with desired properties.
The measurement system can include a first camera and a second camera for stereoscopic imaging of the plasma jets. Each camera has a field of view that includes one or more plasma jets. When two jets are in the field of view of a camera, the camera is position such that throughout the expected range of motion of the plasma jets, an image of one jet remains on one side of a reference point and an image of a second jet remains on the other side of the reference point. The reference point can correspond to an injector of a cold jet so that the plasma jets and the cold jet have desired relative orientations.
Another embodiment of the invention is a method for operating a plasma apparatus that uses first and second plasma jets that converge into a combined plasma stream. The method includes: separately measuring characteristics such as the positions of the first and second plasma jets; and adjusting the first and second plasma jets so that the characteristics of the first and second plasma jets go from the values measured to values previously determined to provide the combined plasma stream with desired properties. When separately measuring the characteristics of the first and second plasma jets identifies the positions of the first and second plasma jets, adjusting the first and second plasma jets includes shifting the first and second plasma jets from the measured positions to positions previously determined to provide the combined plasma stream with the desired properties.
A structure such as an injector of a cold jet can define a reference point for measurement of the separate jets. In one embodiment of the invention, a calibration process mounts a fixture on an injector. The injector is below the field of view of the measurement system but the fixture extends into a field of view of the measurement system. For example, when the measurement system employs cameras, the fixture is mounted on the injector and directs one or more light beams at each camera. The cameras in turn identify the position of the beams and infer the relative position that the cold jet will have during operation of the plasma treatment system. The fixture is then removed for operation of the plasma treatment system.